The present disclosure relates to light guides and illumination devices and particularly relates to a technique for reflecting light, which has entered a light guide, toward the exit surface of the light guide.
Some image forming apparatuses, such as multifunction peripherals, employ as a light source section for an image reading device, such as a scanner, a line light source in which a rod-shaped, resin-made light guide is combined with an LED capable of emitting light to the interior of the light guide through a longitudinal end surface of the light guide. The light source section is required to linearly illuminate an original document to be read in synchronism with reading of a line sensor. For this purpose, a surface of the light guide opposite to an exit surface thereof through which light exits the light guide is provided with light reflecting or scattering patterns, so that the patterns can deflect light, which has entered the light guide, toward the exit surface to allow linear illuminating light to exit the light guide in the direction toward the original document.
However, with the above combination of the rod-shaped light guide with the LED, illuminating light emitted from different locations on the light guide and its exit surface in the longitudinal direction cannot have evenness because the direct light directly deflected by the light reflecting or scattering pattern surface after the incidence from the LED is different in illumination intensity from the indirect light totally reflected once or more by the outer periphery of the light guide and then deflected by the light reflecting or scattering pattern surface. To cope with this, for example, an illumination unit is proposed in which the light guide is not circular but polygonal in cross-sectional shape and is deformed in the main scanning direction to avoid that light from the LED may directly enter the light reflecting or scattering pattern surface.
Furthermore, considering only the indirect light, angled light beams are more likely to be totally reflected by the outer peripheral surface of the light guide and more likely to enter the light reflecting or scattering patterns and then exit the light guide. Therefore, there arises a problem in that with increasing distance from the incident surface of the light guide in the main scanning direction (the direction of the optical axis of light incident on the light guide from the light source section), the angle of light beam from the LED becomes smaller and, thus, the illumination distribution of indirect light in the sub-scanning direction (the direction perpendicular to the direction of the optical axis) during illumination onto the original document becomes narrower. As a solution to this problem, for example, an illumination unit is proposed in which the outer peripheral shape of the light guide is changed overall or locally in the longitudinal direction to vary with location the angular distribution of light beams incident on the light reflecting or scattering patterns, so that the illumination distribution of indirect light in the sub-scanning direction can be equalized over the entire region in the main scanning direction.